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Journal Articles

X-ray photoelectron spectroscopy for ${it in situ}$ observation of surface reactions under the gas atmosphere; History, applications, issues, and future prospect

Takakuwa, Yuji*; Ogawa, Shuichi*; Yoshigoe, Akitaka

Hoshako, 35(3), p.158 - 171, 2022/05

Ambient pressure X-ray photoelectron spectroscopy for in situ observation of surface reactions using high-brightness synchrotron radiation shows a rapid progress in the number of endstations since about 2005 and is applied to various practical research field for clarifying reactions at solid/gas interfaces of e.g. catalyst, solid/liquid interfaces of e.g. fuel cell, and gas/liquid interfaces of e.g. ion liquid. In this review, a history of the development of APXPS, real-time observation of the surface reactions for Si chemical vapor deposition and Si dry oxidation, issues of APXPS, and future prospects are described.

Journal Articles

Oxidation mechanisms of hafnium overlayers deposited on an Si(111) substrate

Kakiuchi, Takuhiro*; Matoba, Tomoki*; Koyama, Daisuke*; Yamamoto, Yuki*; Yoshigoe, Akitaka

Langmuir, 38(8), p.2642 - 2650, 2022/03

https://doi.org/10.1021/acs.langmuir.1c02711
 Times Cited Count：1 Percentile：13.58(Chemistry, Multidisciplinary)

0xidation processes at the interface and the surface of Si(111) substrate with thin Hf films were studied using photoelectron spectroscopy in conjunction with supersonic oxygen molecular beams (SOMB). The oxidation starts at the outermost Hf layers and produces stoichiometric HfO$$_{2}$$. Hf silicates (Hf-O-Si configuration) were generated in the vicinity of the HfO$$_{2}$$/Si interface in the case of the irradiation of 2.2 eV SOMB. The oxidation of the Si substrate takes place to generate SiO$$_{2}$$ compounds. Si atoms were emitted from the SiO$$_{2}$$/Si interface region underneath the HfO$$_{2}$$ overlayers to release the stress generated within the strained Si layers. The emitted Si atoms can pass through the HfO$$_{2}$$ overlayers and react with the impinging O$$_{2}$$ gas.

Journal Articles

Two-step model for reduction reaction of ultrathin nickel oxide by hydrogen

Ogawa, Shuichi*; Taga, Ryo*; Yoshigoe, Akitaka; Takakuwa, Yuji*

Journal of Vacuum Science and Technology A, 39(4), p.043207_1 - 043207_9, 2021/07

https://doi.org/10.1116/6.0001056
 Times Cited Count：1 Percentile：7.45(Materials Science, Coatings & Films)

Nickel (Ni) is used as a catalyst for nitric oxide decomposition and ammonia production but it is easily oxidized and deactivated. Clarification of the reduction process of oxidized Ni is essential to promote more efficient use of Ni catalysts. In this study, the reduction processes were investigated by in situ time-resolved photoelectron spectroscopy. We propose a two-step reduction reaction model. The rate-limiting process for the first step is surface precipitation of O atoms and that of the second step is dissociation of H$$_{2}$$ molecules.

Journal Articles

Dynamic observation and theoretical analysis of initial O$$_{2}$$ molecule adsorption on polar and $$m$$-plane surfaces of GaN

Sumiya, Masatomo*; Sumita, Masato*; Asai, Yuya*; Tamura, Ryo*; Uedono, Akira*; Yoshigoe, Akitaka

Journal of Physical Chemistry C, 124(46), p.25282 - 25290, 2020/11

https://doi.org/10.1021/acs.jpcc.0c07151
 Times Cited Count：11 Percentile：47.05(Chemistry, Physical)

The initial oxidation of different GaN surfaces [the polar Ga-face (+c) and N-face (-c) and the nonpolar (10$$bar{1}$$0) ($$m$$)plane] under O$$_{2}$$ molecular beam irradiation was studied by real-time synchrotron radiation X-ray photoelectron spectroscopy and DFT molecular dynamics calculation. The results predict that triplet O$$_{2}$$ either dissociates or chemisorbs at the bridge position on the +c-surface, while on N-terminated -c-surface the O$$_{0}$$2 molecule only undergoes dissociative chemisorption. On the $$m$$-GaN surface, although the dissociation of O$$_{2}$$ is dominant, the bond length and angle were found to fluctuate from those of O$$_{2}$$ molecules adsorbed on the polar surfaces. The computational model including both the surface spin and polarity of GaN is useful for understanding the interface between GaN and oxide layers in metal-oxide electronic.

Journal Articles

Real time observation on oxidation states on Si(001) surfaces induced by translational energy of O$$_{2}$$ using synchrotron radiation photoemission spectroscopy

Yoshigoe, Akitaka; Teraoka, Yuden

Shinku, 45(3), p.204 - 207, 2002/03

https://doi.org/10.3131/jvsj.45.204

no abstracts in English

Oral presentation

Time-resolved observation of molecular adsorbed oxygen at Si(100)-2$$times$$1 surface using real-time synchrotron radiation photoelectron spectroscopy

Yoshigoe, Akitaka; Okada, Ryuta; Teraoka, Yuden; Iwai, Yutaro*; Yamada, Yoichi*; Sasaki, Masahiro*

no journal, , 

Understanding of fundamental aspects of oxidation at Si(100) surface using O$$_{2}$$ gas is essential for fabrication of gate-oxide films with the thickness of atomic-scale in Si-based MOSFET. The study of precursor states prior to dissociative adsorption is important to clarify the oxidation mechanism. We present the time-evolution of molecular chemisorbed oxygen observed by synchrotron radiation real-time photoelectron spectroscopy.

Oral presentation

Sub-monolayer oxides on Ge(100) surface fabricated with pure O$$_{2}$$ gas

Yoshigoe, Akitaka; Okada, Ryuta; Teraoka, Yuden; Yamada, Yoichi*; Sasaki, Masahiro*

no journal, , 

Ge has received much attention as promising substitute material for future electronic devices because of its high carrier mobility. We report synchrotron radiation photoelectron spectroscopy study on the oxides of Ge(100)-2$$times$$1 surface fabricated with pure O$$_{2}$$ at 300 K and its incident energy dependence. We found that saturated coverage is less than one monolayer. We reveal that Ge oxidation state in saturation is +2 at most, which exhibits a strong contrast to Si. The Ge$$^{2+}$$ population and oxygen coverage increased with incident energy. Our findings are potentially useful for precisely fabricating Ge oxides with submonolayer thickness.

Oral presentation

Strain-induced oxide decomposition at SiO$$_{2}$$/Si(001) and SiO$$_{2}$$/Si(111) interfaces studied by X-ray photoelectron spectroscopy and scanning tunnelling microscopy

Tang, J.*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Teraoka, Yuden; Takakuwa, Yuji*

no journal, , 

Thermal decomposition of SiO$$_{2}$$ layers formed on Si substrate is well known to proceed via the inhomogeneous void nucleation and subsequent 2D enlargement. However, the void nucleation mechanism is still not clear yet. In this study, the thermal decomposition kinetics of SiO$$_{2}$$/Si(001) and SiO$$_{2}$$/Si(111) were observed in real time by X-ray photoelectron spectroscopy using synchrotron radiation at SPring-8 to obtain simultaneously information on the interfacial strained Si atom as well as the oxide thickness and oxidation states.

Oral presentation

Synchrotron radiation real time photoelectron spectroscopy study on oxidation dynamics of Si surface

Yoshigoe, Akitaka

no journal, , 

In this seminor recent study on oxidation dynamics of Si surfaces using synchrotron radiation real time photoelectron spectroscopy is presented and the importance and future perspective of surface chemistry in nanotechnology researches is discussed.

Oral presentation

Strain-induced reaction kinetics of O$$_{2}$$ molecule at SiO$$_{2}$$/Si interfaces studied by real-time X-ray photoelectron spectroscopy

Tang, J.*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Takakuwa, Yuji*

no journal, , 

The reaction kinetics of O$$_{2}$$ molecule at SiO$$_{2}$$/Si interfaces were investigated as a function of O$$_{2}$$ pressure (PO$$_{2}$$) by real-time X-ray photoelectron spectroscopy at BL23SU, SPring-8, to monitor the interfacial oxide growth rate (Rint), the oxidation states (Si$$^{1+}$$, Si$$^{2+}$$, Si$$^{3+}$$ and Si$$^{4+}$$, and the magnitude of oxidation-induced strain (Si$$^{alpha}$$ and Si$$^{beta}$$) at the same time during oxidation. When increasing PO$$_{2}$$ after the surface oxidation, Rint showed a nonlinear power-low function of (PO$$_{2}$$)$$^{n}$$, where n was almost the same as 0.5 for both Si(111) and Si(001) substrates. The results are interpreted by a Si oxidation model, in which the defect generated by the oxidation-induced strain is an active site for dissociative adsorption of O$$_{2}$$ at SiO$$_{2}$$/Si interface. Furthermore, when increasing PO$$_{2}$$ during the surface oxidation, Si$$^{alpha}$$ and Si$$^{beta}$$ showed significant changes at the start of interfacial oxidation.

Oral presentation

Rapid temperature oxidation at SiO$$_{2}$$/Si(001) interface studied by real-time X-ray photoelectron spectroscopy; Rapid cooling versus rapid heating

Tang, J.*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Takakuwa, Yuji*

no journal, , 

The kinetics of rapid thermal oxidation (RTO) on Si(001) with O$$_{2}$$ was investigated by real-time X-ray photoelectron spectroscopy to monitor the changes of thermal strains and oxide growth rate at the same time. We have found that rapid cooling from 500 to 100 degrees as well as the rapid heating from 300 to 500 degrees have a significant acceleration effect on the oxide growth rate at the SiO$$_{2}$$/Si(001) interface. The observed acceleration was ascribed to the defects (emitted Si atoms + vacancies) generation due to the thermal strain, which is the dissociative adsorption site of O$$_{2}$$ at the SiO$$_{2}$$/Si interface. Based on the observed changes of Si$$alpha$$ and Si$$beta$$, a RTO reaction model is considered in terms of the oxidation-induced and thermal strains present at the SiO$$_{2}$$/Si(001) interface. In future, it is of practical importance to investigate the effect of the temperature-changing rate and temperature difference on the oxide growth acceleration.

Oral presentation

Detection on molecularly oxygen adsorbate during room-temperature oxidation at Si(100) surface; Synchrotron radiation light shed light on issue over 3 decades

Yoshigoe, Akitaka; Taga, Ryo*; Ogawa, Shuichi*; Takakuwa, Yuji*

no journal, , 

In this conference, the experimental evidence on the existence of molecularly adsorbed oxygen during the oxidation of Si(100)2$$times$$1 surface, which was considered to be absent, is reported. Synchrotron radiation photoelectron spectroscopy measurements was used to reveal its presence during the oxidation at room temperature and -150$$^{circ}$$C.

Oral presentation

Surface chemistry experimental station at Soft X-ray beamline (BL23SU) in SPring-8

Yoshigoe, Akitaka

no journal, , 

Synchrotron radiation based X-ray photoelectron spectroscopy (SR-XPS) is a powerful tool to conduct real time monitoring of surface chemical reactions such as semiconductor manufacturing processes and catalysis. In this symposium, the studies on oxidation mechanisms at the single crystal surfaces of IV-group semiconductor (Si and Ge) materials will be shown as an example to describe the research activities and the potential advantages for surface chemistry experimental station at the soft X-ray beamline (BL23SU) in SPring-8.

Oral presentation

O$$_{2}$$ pressure dependence of SiO$$_{2}$$/Si interfacial oxidation rate studied by real-time photoelectron spectroscopy

Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Takakuwa, Yuji*

no journal, , 

In this study, the increased O$$_2$$ pressure dependence of the interface oxidation rate was investigated using real-tile photoelectron spectroscopy. The experiment was performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. From $$I_{O1s}$$, we can estimate the completion of surface oxidation as 3200 s. Then, O$$_2$$ pressure is increased, and then the interface oxidation is enhanced. The P$$_O2$$ dependence of the interfacial oxidation rate shows that O$$_2$$ pressure increase makes the interfacial oxidation rate fast, and the interface oxidation rate is proportional to the square root of $$P_O2$$.

Oral presentation

Oxygen kinetic energy dependence of the satellite peak observed in O1s photoelectron spectra for Si(001)2$$times$$1 surface oxidation at room temperature

Yoshigoe, Akitaka; Tsuda, Yasutaka; Tominaga, Aki; Sakamoto, Tetsuya; Ogawa, Shuichi*; Takakuwa, Yuji*

no journal, , 

In Society 5.0, nanometer-scaled devices with high performance and low-power are required. Since silicon (Si) based metal-oxide-semiconductor (MOS) transistors play an important role in the application of information and communication technology, precise control and understanding of Si surface oxidation is still important. However, the debate on the Si surface oxidation by molecular oxygen still remains. In our previous study, we found that the O1s photoelectron spectra of room temperature oxidation of Si(001)2$$times$$1 surface show satellite peaks indicating molecularly adsorbed oxygen. In this study, the role of molecularly adsorbed oxygen in the oxidation reaction in relation to the translational energy of incident oxygen molecules was investigated by synchrotron radiation real-time photoemission spectroscopy. All experiments were conducted using the surface experimental station of JAEA's BL23SU at SPring-8. Even on a p-type Si(001) surface under molecular beam irradiation with the translational energy of 0.06 eV, a satellite peak was clearly observed at a binding energy of about 4.45 eV lower than the main peak (oxygen atom in Si-O-Si). The role of molecularly adsorption in the oxidation reaction will be discussed from the results of signal changes with the progress of oxidation times.

Oral presentation

Molecular beams study on satellite peak observed in O1s photoelectron spectra for Si(001)2$$times$$1 surface oxidation at room temperature

Yoshigoe, Akitaka; Tsuda, Yasutaka; Tominaga, Aki; Sakamoto, Tetsuya; Ogawa, Shuichi*; Takakuwa, Yuji*

no journal, , 

The basic understanding of the oxidation at Si surfaces by oxygen gas has been important to develop advanced metal-oxide-semiconductor devices. In our previous study, we succeeded to detect a satellite peak located at lower binding energy sides of the main peak (Si-O-Si) in the Si(001)2$$times$$1 surface oxidation. This peak has been considered to be due to the molecularly adsorbed oxygen. In this study, we studied the incident O$$_{2}$$ energy dependence of this adsorbate by using real-time photoemission spectroscopy. All experiments were conducted at the surface experimental station (SUREAC2000) at BL23SU in SPring-8. It was found that the satellite peak was clearly observed. This result also implies the existence of a precursor state of adsorbed oxygen dissociation even for 0.06 eV.

Oral presentation

Real-time observation of gas-solid surface reactions by using synchrotron radiation photoelectron spectroscopy

Yoshigoe, Akitaka

no journal, , 

Real-time photoelectron spectroscopy studies of solid surface reactions that have been carried out at the surface reaction experimental station at the JAEA soft X-ray beamline (BL23SU) in SPring-8 are presented. This invited lecture will provide an overview of the equipment and techniques used, focusing on the study of semiconductor surface oxidation, and introduce the usefulness of real-time photoelectron spectroscopy using synchrotron radiation, including its application to other reaction systems such as metals. Future prospects for material DX research, which has been rapidly paying attention in recent years, will also be talked.

Oral presentation

Oxidation mechanism proceeding at local/entire area of hafnium absorbed Si(111)

Kakiuchi, Takuhiro*; Tsuda, Yasutaka; Yoshigoe, Akitaka

no journal, , 

HfO$$_{2}$$ has attracted much attention as a high-k gate dielectric material for Si semiconductor devices. In this study, oxidation by O$$_{2}$$ gas exposure (Et: 0.03 eV) and supersonic O$$_{2}$$ molecular beams (Et: 0.39, 2.2 eV) on Si(111) with different amounts of Hf adsorbed at about 0.5 and 2.0 ML were observed and analyzed by Hf4f, Si2p and O1s photoelectron spectroscopy. At low coverage of 0.5 ML, Hf adsorbs on rest-atoms and adatoms on Si(111)-7$$times$$7 to form a peculiar local structure (hexagonal structure), around which oxidation proceeds only to Hf$$^{3+}$$ silicate. At 2.0 ML, on the other hand, the oxidation reaction proceeds rapidly over the entire surface metal Hf layer and is considered to produce up to Hf$$^{4+}$$ silicate.

Oral presentation

Real-time observation of oxidation process on GaN surfaces by X-ray photoelectron spectroscopy

Sumiya, Masatomo*; Tsuda, Yasutaka; Sumita, Masato*; Yoshigoe, Akitaka

no journal, , 

Synchrotron radiation X-ray photoelectron spectroscopy (XPS) was employed to clarify the oxidized states on the polar and m-plane GaN surfaces under exposure of various oxidation gases of H$$_{2}$$O, O$$_{2}$$, N$$_{2}$$O, and NO. It was found that H$$_{2}$$O vapor has the higher reactivity. The oxygen was hardly adsorbed on the surface by irradiating N$$_{2}$$O and NO gases. Apparently, two oxidation states for O$$_{2}$$ and H$$_{2}$$O irradiation were detected on +c GaN surface. Physisorption of O$$_{2}$$ molecule was dominate. The dissociation and adsorption of H$$_{2}$$O molecules co-existed on the +c surface. The chemisorption on the m-plane of GaN was dominant, and a stable Ga-O bond was formed on the surface. These chemical oxygen states were simulated by density functional molecular dynamics calculation using a theoretical model including both electronic spins on the surfaces and the polarity of GaN.

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